How to Determine the Sex of Your Tarantula

The Proper Technique for Sexing Living Theraphosid Spiders

by Mark R Hart
Edited and Reviewed by Rick West

Perhaps the most controversial subject in the captive husbandry of tarantulas is the issue of "how to sex a tarantula". Over my 30 years of keeping tarantulas alive in vivaria, I must admit to having been fooled many times both by the spiders themselves and by my well-meaning colleagues or so called "experts", who claimed to have discovered some foolproof method of accurately determining the gender of individual tarantulas.

Well, if those techniques were indeed "foolproof", then I must be nobody's fool, since they have proven to be about as (but no more) accurate as flipping a coin. Heads is female, tails is male. The author has heard everything (including a few you may have missed!) about how you can accurately determine the sex of a living tarantula. From "feeling" for the abdominal bulge (try THAT one with a King Baboon or Usambara Red!) to comparing the sizes of the chelicera, legs, abdominal girth or the you-name-it. This includes being told, "the bigger one is the female" or male, as the case may be, until my face is bluer than a Haplopelma lividum' s legs! Invariably, the "female" will molt out with tibial hooks and palpal emboli and the much needed "male" that you raised from a spiderling will continue to molt out every year with nary a sign of maleness. All this from the prophets of theraphosid prognostication, who SWORE that the spider must be a male because its "such and such was so and so". Alas, was there no True Science in the muddled forest of Pseudoscientific methods and outright chicanery??

Fortunately, for those of us who breed tarantulas regularly, the technique of sexing theraphosid spiders from the microscopic examination of the exuviae (shed skins), is a True science. Our colleagues in the British Tarantula Society, especially John and Kathleen Hancock, brought this valuable technique for accurately sexing tarantulas to light in their book entitled, "Tarantulas: Keeping and Breeding Arachnids in Captivity" by R & A Publishing Ltd., Somerset, England. Anyone with access to a light microscope could be fairly certain of the sex of their tarantulas when an exuvium became available. This technique is very important, and one which every serious tarantula breeding MUST learn, to ensure the long term planning and availability of males for future breeding projects. Unfortunately, this technique has its limitations. I mean, what does one do when standing in a pet store or at a breeder's table at a show, with onlookers pressing up against you? Try saying "Excuse me, Mr. Pet Store Owner, did you happen to save the last shed skin from this particular Brachypelma smithi? Oh, and do you happen to have a stereoscopic microscope that I might borrow for a moment?" Even if one were to get so lucky, you might always doubt that the skin actually came from the spider in question. Also, in home collections, even for those who keep meticulous records, it is possible to get skins confused when taking dozens of molts from dozens of cages over dozens of days.

Thus the quest remained for a reliable method based on scientific evidence that allows one (sending out DNA samples was not reasonable) to immediately determine the gender of a live tarantula. Pouring over endless volumes of foreign texts, in Germany, French and Portuguese, had led me to the conclusion that they might have stumbled onto some technique. Further investigation (and expensive translation software!), unfortunately, revealed that the many foreign authors had not found any ONE characteristic that they could agree on. All were merely educated guesses, based on many, many years of experience and looking at many thousands of spiders, most of the same species or two. Certainly, if one has 200 tarantulas of the same species, of nearly the same age & size, it is possible to select males from females with a better than 50/50 chance. But what about the lone tarantula in a "Critter Cottage" in the corner of a pet shop on Main Street? Or the lone specimen of a rare species that you need to pair up and there is only one large juvenile in the dealer's display case and he swears that it is the exact sex you need? Where was Science then, to help prove the dealer to be honest or expose the truth? But this endeavor was not to go unrequited forever.

During the long, hot summer of 1995, I was fortunate enough to attend the "Invertebrates in Captivity" Conference of the Sonoran Arthropod Studies Institute, held annually in Tucson, Arizona. The Director of SASI, Steve Prchal, had assembled what was to be a tarantula lover's Dream Event. In attendance where such noted tarantula specialists as Andrew Smith of the British Tarantula Society and noted author, Barbara Reger, Dr. Bob Breene and Miep O'Brien of the American Tarantula Society, Tom Mason of the MetroToronto Zoo, David Hodge of the Louisville Zoo, Dr. G.B. Edwards of the Florida Department of Agriculture, Dr. Robert Wolff of Trinity College and a list of others too numerous to mention. Surely the answer to the timeless question of theraphosid gender determination would be answered definitively here, and we were NOT to be disappointed.

Enter Rick West of Victoria, British Columbia. Rick had been coaxed from his home in the idyllic Canadian Northwest to come and give a two day "mini Seminar" on theraphosid spiders. While the SASI conference covers ALL Arthropods, most of the over 100 attendees were, at the very least, passingly interested enough in tarantulas to make the seminar a worthwhile event on its own. Rick's seminar was to cover two main areas. The first was the presentation of his illustrated Keys to help hobbyists and invertebrate specialists identify theraphosid subfamilies (and some genera) and the second part was to demonstrate the easy method of sexing living tarantulas.

For those not familiar with Rick West or his work, Rick is one of those rare individuals whose thirst for personal knowledge has caused him to acquire vast numbers of sometimes obscure papers on various things arachnological. This, coupled with Rick's own encyclopedic knowledge of the subject of tarantulas was to lead to this breakthrough in the community of tarantula enthusiasts.

The very first item passed out by Rick was a copy of a paper entitled, "The Spinnerets and Epiandrous Glands of Spiders" by B. J. Marples, J. Linneaus Society (Zool.), vol. 46, 1967. This paper is about spider spinnerets, of all things, and made references to certain fusilla or spinnerets that are found only on male spiders, called epiandrous fusillae. According to the paper, nearly all male spiders, including all tarantulas, have this extra set of silk spinning glands and fusillae or spinnerets. If only we could find them on a spider, we could at last determine, conclusively and through Science, whether the spider was male or not male (i.e., female).

Apparently, almost all male spiders, including all theraphosids, have this extra set of silk spinning fusules or tubes that are connected to epiandrous glands. It is believed the epiandrous glands produce a special silk used in the construction of the sperm web, a structure only built by male spiders in aid of transferring seminal fluid from the genitalia to the palpal bulbs. In addition to this paper was a page of illustrations presented and reproduced, in part, by Mr. West from ‘Zur Biologie Vogelspinnen (Fam. Aviculariidae)’, M. Melchers, Z. Morph. Okol. Tiere 53, 1964. These illustrations more clearly showed the location and appearance of the structures in question on a tarantula. As can be seen by the drawings, and in conjunction with the text by Mr. Marples, the epiandrous fusillae are positioned on the central anterior edge of the males epigastric furrow.

A detailed chart on sexing tarantulas may be downloaded here. - [PDF 506KB]

(This file is in Acrobat PDF format, to downloaded it you will need the Acrobat Reader. You may download it for free from here.)

They occur in between 2 and 4 rows and form a semi-circular or sometimes triangular shape that suggests an "arch" of more dense, shorter, darkened hairs or setae. The author was in attendance at this seminar with many hundreds of live tarantulas of numerous species and we didn’t hesitate to lay hands on many unsuspecting victims for our "hands on" approach to determining their gender.

The Aphonopelma chalcodes Chamberlin, 1940 pair pictured above are about the same size and shape, however one of them is an about-to-mature (penultimate) MALE. Care to take a guess? Let's turn them over and take a look.

On larger spiders, the presence (or absence) of the epiandrous fusillae is generally visible to the naked eye, although a good light source is recommended.

With smaller tarantulas, down to a size that can be safely picked up and turned over (i.e., 1.5-2" legspan), the author prefers to used a pocket microscope of about 30x. There is a handy one available, WITH a built-in light, from Radio Shack® for about $10.00 US. By placing the ‘scope' flush against the "belly" of the spider to be examined, one can readily view the presence or absence of this rather distinctive sexual characteristic of the male tarantula. Further examination of molted skins reveal that the epiandrous fusillae are visible on even very small spiderlings.

This pair of Nhandu coloratovillosus [= Brazilopelma coloratovillosum] (Schmidt, 1998) are juvenile spiders with legspan of about 3 inches or so. One is male and the other female. Shall we look and see? These spiders also have light colored undersides.

One has to ask, then, how did the tarantula-keeping community overlook such an obvious characteristic for so long? Though it is impossible to know with certainty, one might hazard a guess. It is undoubtedly true that the average tarantula keeper (not breeder) is almost always on the lookout for a FEMALE tarantula. The fact that they are larger, more massive and live far longer in captivity all add up to the choice of a female being the wiser decision and choice over a male. Except for breeding, the male is a poor financial investment for the average tarantula keeper. This may have led, in the author's opinion, to a condition of ‘seeing only what we are looking for’ and ‘if we are looking for females, let’s try to find what females have that males don't’.

This Mexican Fireleg appears to be a female; no hooks or palpal emboli. But let's take a look underneath...

This is the view from underneath. What do you think now?

In fact, in the first scientific spider sexing research, the microscopic examination of spider molts is done with the primary goal of identifying the presence of the spermatheca or ‘sperm pocket’ of female spiders. This work, in trying to determine females, may have led to a case of ‘missing the forest because of the trees’!

The visual examination of even very small (2nd to 3rd instar) tarantula exuviae show the epiandrous ‘arch’ of fusillae as readily or more readily than the developing spermatheca of very young female tarantulas and, perhaps just as importantly can be seen within seconds on the live animal! The magnified photos figured in this article should help illustrate the appearance and location of these silk-producing hairs that appear like short, stiff and slightly bent hairs. Under high magnification, these hairs take on the shape of an arch (or inverted triangle in some species such as Pamphobeteus and Megaphobema). On the underside of some tarantula species, such as Aphonopelma seemanni (F.O.P.-Cambridge, 1897) or Megaphobema robustum (Ausserer 1875), the fusillae can be the same color as their surrounding pile or covering hairs.

Compare the female on the left with the male Vitalius wacketi [= Vitalius platyomma] (Mello-Leitao, 1923) on the right. Click the images to enlarge.

With some practice and a pocket microscope for the smaller spiders, sexing tarantulas can be simple. Use care when handling your spiders! The gender of your spider is moot if you kill it in the process.

On these aforementioned tarantula species, they have light orange to peach colored epiandrous fusillae, which blend with the same surrounding colored hairs. However, it’s the placement, shape and number (2 to 4 rows) of these fusillae which are important and NOT the color. This error could easily cause one to look upon all hairs on the anterior epigastric region as epiandrous fusillae, erroneously leading one to believe all tarantulas are males.

Naturally, time and practice are required. If done on a live tarantula, such as a Haplopelma or Pterinochilus or any of a number of fast and aggressive tarantulas, there are inherent risks of an entirely different nature, which require a modicum of skill and a pair of forceps. The author uses forceps clamped ventrally along the carapace between the #2 and #3 legs. We recommend a firm grip and a prayer! Many of the larger arboreal species can be positively sexed while they are posed against a glass or plastic wall, given sufficient light, a keen eye and a magnifier.

In the beginning, you are apt to make a mistake or two and the level of ones eyesight (mine is rather poor) can also add to or detract from the success you will have with this method. So, the differences are on every tarantula you’ll ever look at. Take the time and work on it, if you care. Don’t expect the pet store clerk or the tarantula dealer (myself included) to pick up and sex every tarantula for you. Learn to do it yourself! If the dealer chooses to do so, it’s just a courtesy, especially for large numbers of small spiders. Additionally, one has to consider the ‘itch factor’ from New World theraphosids and the ‘chomp factor’ from the Old World species. Perhaps one day, sexing for a fee will be the rule of the day, but until then, you now have the tools to go ‘look up a few kilts’ and tell, once and for all, exactly whether you are looking at a male or a female tarantula. Of the several hundreds (perhaps thousands) of live tarantulas the author has sexed since the 1995 demonstration by Rick West, there've been very few challenges as to the accuracy of this tarantula sexing method. Not ONE spider, upon it’s maturing molt, has proved to be the opposite gender and been verified by subsequent microscopic examination of the molted skin (yes, the author checks this regularly). It’s scientifically based and it’s a method that works. Once you have perfected this technique, you’ll be able to sex tarantulas as easily as you can sex a dog and even easier than most cats! So get to work!

The images on this website are merely a small cross-section of the species
and images found in Rick West's photo collection. Viewers are welcome to
use this website as an identification tool, however, viewers must realize
that although many tarantulas are colour/pattern specific, identifications
can never be positively made from images alone. Additionally, although Rick
West keeps up with taxonomic changes scientific names change constantly and
may not always be immediately reflected on this website.